A sample of ammonia gas initially at 400 K and 1.0 atm is slowly subjected to…
CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
A sample of ammonia gas initially at 400 K and 1.0 atm is slowly subjected to simultaneous cooling and compression until it reaches 240 K and 150 atm. The container is equipped with sensors that continuously record the compressibility factor Z=nRTPV.
The data show that Z first decreases below 1.0, reaches a minimum of 0.82, then rises above 1.0 during the final stages of the process.
(a) Describe qualitatively why Z drops below 1.0 during the early portion of the process. In your answer, identify the dominant molecular-level effect and relate it to the macroscopic observation.
(b) Explain why Z eventually increases above 1.0 at high pressure even though the temperature continues to decrease.
(c) Estimate the approximate pressure range over which attractions dominate in this experiment, based only on the trend in Z, and justify your reasoning.
(d) The minimum value of Z occurs at 0.82. Predict how the depth of this minimum would change if the same experiment were repeated starting at 500 K instead of 400 K, and give a molecular explanation for your prediction.